Don't detach xfr in dns_xfrin_shutdown()

Don't detach xfr in dns_xfrin_shutdown()

As we are now using dispatch instead of netmgr for XFR TCP connection,
the xfrin_recv_done() will be called when cancelling the dispatch with
ISC_R_CANCELED.  This could lead to double detach from the dns_xfrin_t,
one in the xfrin_recv_done() and one in the dns_xfrin_shutdown().

Remove the extra detach from the dns_xfrin_shutdown() and rely on the
dispatch read callback to be always called.